This invention relates to crosslinkable or chain extendable polyarylpolyamines, methods for the preparation of such crosslinkable or chain extendable polyarylpolyamines and films thereof. The films of the polyarylpolyamines are useful as charge transport layers in light-emitting diodes.
Triarylamines, as evidenced by their low oxidation potentials, are easily oxidized to the corresponding radical cations. The cations are equally easily reduced to the neutral starting amines. This oxidation/reduction process is reversible and can be repeated many times. For this reason, triarylamines are widely used as charge transport materials specifically for the transport of holes (positive charges).
Charge transport materials are essential to the efficient operation of electrophotographic devices (copying machines and printers) and electroluminescent devices such as light-emitting diodes. In both applications, the triarylamines are used in film form. For electrophotographic applications, a triarylamine and a polymeric binder are dissolved in a suitable solvent and the resulting solution used for coating, see U.S. Pat. Nos. 5,352,554 and 5,352,834. Polycarbonates, polystyrene, poly(vinylcarbazole), poly(vinylbutyral) and poly(methyl methacrylate) are some of the polymers used as binders. To obtain a film of useful charge transport properties, the loading of triarylamine in the final formulation must be as high as possible, preferably more than 30 percent of the total formulation with 50 percent by weight loading levels common. In low concentrations, the triarylamine will act to trap charge carriers instead of transporting them, D. M. Pai, J. F. Yanus, M. Stolka., J. Phys. Chem., Vol. 88, p. 4714 (1984). The triarylamine compound must be soluble in high concentrations in the binder polymer after the film is formed and the solvent is removed. If the triarylamine compound separates out from the polymer binder or crystallizes into a fine dispersion of crystals in the polymer binder, the film can no longer serve its intended purpose.
Organic electroluminescent devices are typically constructed by sandwiching an organic film or a stack of organic films between an anode and a cathode such that when voltage is applied, holes and electrons are injected and transported into the device. The combination of holes and electrons within the organic layer leads to excitons which can undergo radiative decay to the ground state, emitting the excitation energy in the form of light. For the light to be seen, it is necessary that one of the electrodes be transparent. Mixed metal oxides, particularly indium tin oxides (ITO), form smooth, conducting, transparent films and are most commonly selected as the anode material. In practice, a sheet of ITO-coated glass is used as the substrate and onto the ITO side is deposited an organic film, and onto this film is deposited a second metal as the cathode. The cathode material is a metal of lower work function than ITO. Metals such as calcium, magnesium, indium and aluminum are used. A major improvement in device efficiency was achieved when a film of a triarylamine was deposited by conventional vapor-phase deposition between the emitting film and the anode, see C. W. Tang, S. A. Van Slyke, Appl. Phys. Lett., Vol. 51, p. 913 (1987), and U.S. Pat. No. 4,539,507. One of the problems associated with devices of this type is the tendency of the organic films to crystallize due to the heat evolved during operation, see C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett., Vol. 56, p. 799 (1990). Contacts between organic layers and electrodes may be destroyed by crystallization, leading to device failure, see J. Kido, M. Kohda, Appl. Phys. Lett., Vol. 61, p. 761 (1992).
European Patent Application 372 979 teaches hole-transporting polymers of the structure ##STR1## prepared from the reaction of aldehydes and aromatic amines. These polymers are freely soluble in common organic solvents.
Copolymers consisting of aromatic amide and triarylamine groups have been claimed as hole-transporting layers in electroluminescent devices, see Japanese Patent 0531163-A. These copolymers are less desirable for use in electroluminescent devices as the concentration of the active triarylamine groups are depressed by the presence of the amide comonomer.
Burroughes et al. disclosed that an organic polymer can be used as the emitting layer, see J. B. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature, Vol. 347, p. 539 (1990) and U.S. Pat. No. 5,247,190. Since polymer films can be formed on substrates by a variety of solution coating techniques, and they are much more robust mechanically than vapor-deposited films of small molecules, this discovery is seen as enabling the development of large-area displays.
To construct an electroluminescent device from a polymer film, it would be advantageous to have available hole-transporting materials that can be crosslinked to solvent-resistant polymer films so that a second polymer film can be applied in the form of a solution without affecting the integrity of the first polymer film.
EP 0637899 discloses an electroluminescing arrangement containing one or more organic layers, characterized by at least one of the layers being obtained by thermal or radiation-induced crosslinking and by the fact that it contains at least one charge-transporting compound per layer. Disclosed are known, relatively low molecular weight, charge-transporting compounds which carry anionically, cationically or radically polymerizable groups. Among the known charge-transporting compounds are tertiary aromatic amines, oxadiazoles, thiadiazoles, benzoxazoles, benzotriazoles, phthalocyanines, condensed aromatic systems such as perylenes, pyrenes or coronenes or polyene compounds. Radically polymerizable groups disclosed are vinyl carbonyl compounds such as acrylates, methacrylates or maleic acid derivatives. Cationically polymerizable groups are groups which react with protic acids or Lewis acids to form polymers and include vinyl ether and epoxide groups. Anionically polymerizable groups include cyanoacrylate, methacrylate or styrene. The compounds disclosed require large amounts of energy to result in excitation sufficient to cause light emission, that is, from 87 to 93 volts.
What is needed are relatively high molecular weight charge transport compounds which are capable of forming films and can be crosslinked by conventional methods, such as by radiation or heat-curing, to form solvent resistant films. What is needed are such compounds which can form such films that can maintain their integrity when exposed to a solution of another organic polymer. What is also needed are such compounds which do not require high energy input levels to emit light. What is also needed are polymeric light-emitting diode devices prepared from such films. What is also needed are processes for the preparation of such compounds.